1. Field of the Invention
The present invention relates generally to prosthetic devices and more particularly to a permanent prosthetic ligament.
2. Description of the Prior Art
Attempts to repair or replace damaged ligaments have been varied and generally inadequate for immediately restoring full strength and stability to the involved joint. Workers in the field have transferred natural tissue from other parts of a patient's body to the involved joint. Synthetic materials have also been used to augment natural tissue transfers. A number of techniques employing carbon fiber-type or polypropylene augmentation devices are described in 196 CLINICAL ORTHOPAEDICS AND RELATED RESEARCH, SYNTHETIC LIGAMENTS AND TENDONS, (H. Alexander & A. Weiss eds. June 1985). For example, a flat strap-like braid of polypropylene fibers was used to augment natural tissue grafts in studies conducted on goats. See G. McPherson et al. "Experimental Mechanical and Histologic Evaluation of the Kennedy Ligament Auogmentation Device", CLINICAL ORTHOPAEDICS supra at 186. The time required for the recipient to return to normal activity is generally about one year or longer.
As an alternative to natural tissue grafts, xenografts, tissue grafts from a species other than the recipient species, have been implanted to replace natural ligaments. Like the natural tissue grafts and the augmentation devices, xenografts have tended to be unpredictable in the long term for restoring full strength and stability to the involved joint.
Another type of prosthetic ligament relies on bone ingrowth to aid in the attachment of the ligament to bone. Bone growth strengthens the attachment but requires about six months to complete. In the meantime, the recipient's mobility should be restricted. Dahlen et al. U.S. Pat. No. 4,187,558, which issued on Feb. 12, 1980, describes a flexible braid made of polyethylene terephthalate, encased in silicone rubber. A velour covered collar at one or both ends of the braid aids in attachment to the bone and promotes bone ingrowth to anchor the device.
Several permanent, nonaugmented prosthetic ligaments have been developed. Dore U.S. Pat. No. 4,301,551, which issued on Nov. 24, 1981 describes a deformable silicone core surrounded by a tensionable wrapping of stainless steel threads wound in a helical angle about the core. The core is the load bearing member and is capable of large elastic deformation in response to compression by the threads when the device is stretched. Two rigid plastic rods, one at each end of the core, connect the device to the bones of a joint.
Treace U.S. Pat. No. 3,953,896, which issued on May 4, 1976 describes a prosthetic ligament made of a flexible high molecular weight polyethylene rod. Stainless steel sleeves and polyethylene nuts on each end of the flexible rod hold the prosthetic ligament to the bones.
A third permanent, nonaugmented prosthetic ligament reported by C. Bolton and W. Bruchman, "The GORE-TEX.TM. Expanded Polytetrafluoroethylene Prosthetic Ligament", CLINICAL ORTHOPAEDICS supra at 202, is constructed of bundles of Gore-Tex.RTM. fibers arranged in a braided configuration. The braid is fixed by bone screws placed through eyelets at each end of the braid.
The goal of ligament replacement is to permit the recipient to return to his or her full range of activity as soon as possible. To that end, researchers have attempted by several means to mimic some of the parameters of strength, flexibility, extension and/or recovery found in natural ligaments. Natural ligaments are bands of flexible fibrous connective tissue which join bones or hold organs in place. The mechanical properties of a natural ligament were reported by D. Butler et al., "On the Interpretation of Our Anterior Cruciate Ligament Data", CLINICAL ORTHOPAEDICS, supra at 26. Butler et al. measured the stiffness of the anterior cruciate ligament-bone unit. The stiffness measured for young donors was 182.+-.56 kilonewtons per meter and for older donors was 129.+-.39 kilonewtons per meter. The maximum force for tissues obtained from young donors was 1730.+-.66N.
An object of the present invention is to provide a permanent nonaugmented prosthetic ligament having parameters of strength, flexibility, extension and recovery that at least approximate those of a natural ligament. A further object of the present invention is to provide such a prosthetic ligament that does not depend solely upon bone ingrowth for strengthening the attachment to the bones of the involved joint, and thus, does not require long periods of recuperation before the recipient can resume a full range of normal activity.